THE ALGAE 55 



CHLOROPHYCEAE 



The Chlorophyceae are Algae in which the only pigment is chlorophyll. 

 It is contained in special bodies within the cells, which are termed chloro- 

 plasts. The shape and arrangement of these chloroplasts varies greatly in the 

 different groups, and is frequently used as a means of classification. Among 

 higher plants the chloroplasts are remarkably uniform in shape, but it is 

 evident that the Green Algae are still in an experimental stage of evolution 

 in this respect and that fixity of form has not been attained. 



The lower members are unicellular or colonial. The higher members 

 are mostly filamentous, but a few form flat thalli consisting of cells which 

 divide in two planes. In certain types the body of the organism consists of a 

 single giant cell containing many nuclei ; such a structure is termed a 

 coenocyte. Occasionally the thallus may be made up of a number of 

 coenocytes embedded in mucilage. 



The Green Algae occur mostly in fresh water, though a few are found in 

 the sea, and some occur on land, but generally in moist situations. In fact 

 quite a large number have been isolated from the soil, though all of very 

 simple types. 



The method of reproduction varies considerably. In some species both 

 sexual and asexual reproduction occurs, the former either by isogametes 

 or by complex antheridia and oogonia ; the latter by small motile bodies 

 termed zoospores. This name recalls the belief of early naturalists that 

 in producing these motile cells the plant was actually turning into an animal. 



The Chlorophyceae are divided into a number of Orders, of which we 

 shall consider types from the following : — • 



1. Volvocales {e.g., Chlamydomonas, Haematococcus, Gom'um, Pandorino, 



Eudorina, Volvox). 



2. Chaetophorales {e.g., Pleurococcus, Coleochaete). 



3. Ulotrichales {e.g., Ulothn'x). 



4. Oedogoniales {e.g., Oedogonium). 



5. Conjugales {e.g., Spirogyra).^ 



6. Siphonales {e.g., Bryopsis, Wuicherin). ,-^ 

 Siphonocladiales {e.g., CladopJwra). 



Volvocales 



The Volvocales are Chlorophyceae in which the thallus is unicellular or 

 colonial, and is generally motile throughout life. In the colonial forms 

 separate cells may be embedded in a common envelope of mucilage, and the 

 cells may be systematically linked together by fine protoplasmic threads. 

 The vegetative cells possess extremely fine thread-like flagella, which project 

 through the wall of the cell and are capable of active motion. In the Volvo- 

 cales each cell is generally provided with a pair of flagella of equal length, 



